Facts, Opinions, and Theories.

A fact is something which is objectively true.  Two plus two is four.  The earth is roughly spherical.  President Obama was born in Hawaii in 1961, roughly two years after it became a state, to a mother born in Kansas.  There are also false facts: things which are objectively false.  Two plus two is seven.  The earth was created roughly 6000 years ago.  Liberalism is the same thing as communism and is also the same thing as fascism.   The founders were a pack of gun toting anti-government, anti-tax, bible thumping activists.

An opinion is an attitude or belief about something which may be difficult to validate, or about which your opinion is really of no consequence to anybody but yourself.  I think there is probably life elsewhere in the universe.  Blue is a better color than Red.  Fangio was a better driver than Schumacher.  You can have opinions about facts.  It's sad that our bodies wear out as we get older.  It's annoying to pay taxes.   An opinion is sort of a middle ground, between true and false facts.  Opinions are sometimes validated or invalidated as we learn more about things. 

Since there are a lot of things that are difficult to validate, a lot of people have opinions which may actually be facts or false facts.  For example, you may think that Obama is a Kenyan terrorist.  You are entitled to think anything you like of course, but if that's your belief, you are simply wrong, and acting on that belief in some way may be harmful.  If you think that government tightening its belt during an economic downturn is an unquestioned necessity, you are simply wrong.  It may be a little too complex for a lot of people to understand.  So was the roundness of the earth before circumnavigations or satellite photos.   But lots of people did understand that and knew the truth in the face of popular belief to the contrary.

A theory is an idea about the way some particular thing works in the universe.  Theories can be true, false or opinion.  The theory of the flat earth proved to be false.  The theory of evolution proved to be true.  The theory of universal gravitation is basically true, but it turns out to be more complicated than that.  The theory that there is life elsewhere in the solar system remains possible but is as yet unproven one way or the other, so having an opinion either way is reasonable.  The word "theory" is sometimes used to cast aspersions on a fact that the speaker is unhappy about.   Not liking a theory has no bearing on whether it's true or false, nor does misunderstanding what the word means. It may have an impact on the politics around a theory, which may affect funding for research or even lead to ostracism.  Thinking that something true is false or vice versa, is likely to lead you to make strategic errors.